One common component of an orthogonal frequency division multiplexing (OFDM) system is the cyclic extension of each OFDM symbol. A cyclic extension, or guard interval, may be produced by replicating part of the OFDM symbol and either prepending or appending the replicated segment onto the original symbol. When attached to the frontal portion of the symbol, the cyclic extension may be referred to as a cyclic prefix extension. When attached to the terminal portion of the symbol, the cyclic extension may be referred to as a cyclic postfix extension. For example, given a set of 64 samples, the cyclic extension may be 16 samples. In the case of a prepend, the last 16 samples of the original 64 samples may be prepended to the front of the original 64 samples to result in an 80 sample symbol. In the case of an append, the first 16 samples of the original 64 samples may be appended to the end of the original 64 samples to result in an 80 sample symbol. In either case, taking 64 samples of the 80 sample symbol will result in the original 64 samples by cyclically wrap at the end back to the front if needed. The cyclic extension of an OFDM symbol converts the linear channel convolution into a cyclic convolution. An OFDM receiver exploits the properties of the cyclic convolution to mitigate inter-symbol interference and inter-carrier interference.
While the cyclic extension helps to mitigate channel distortion in an OFDM system, using a cyclic extension involves the transmission of duplicate information, and as such represents an overhead associated with an OFDM transmission scheme. Furthermore, to mitigate the effects of the channel temporal dispersion, the cyclic extension typically is required to be of a duration greater than or equal to the channel memory. For example, in a wireless local area network (WLAN) compliant with an Institute of Electrical and Electronics Engineers (IEEE) standard, such as the IEEE 802.11a standard, the cyclic extension is a predetermined length, for example 800 ns, in order to provide immunity to delay spreads up to a maximum of 800 ns. However, under typical conditions, the actual delay spread experienced is often much lower than the predetermined delay spread of 800 ns, thereby resulting in an unnecessary overhead represented by the difference between the predetermined delay spread and the actually experienced delay spread. It would therefore be desirable to provide an OFDM system in which the length of the cyclic extension may be dynamically allocated according to the actually experienced delay spread, rather than being based on the predetermined, maximum, worst case scenario delay spread. Furthermore, since the actually experienced delay spread may be different for different users, it would in addition be desirable to provide a cyclic extension length for each user based upon the actual delay spread experienced by each respective user.
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